This invention relates to water-soluble sulfonated melamine-formaldehyde resins which may be used as superplasticizers for concrete and for other purposes. More particularly, the invention relates to processes for producing such resins.
Concrete superplasticizers are products that, when added to concrete mixes, provide increased fluidity without the need for additions of extra water that tend to prolong curing times and may weaken the strength of the resulting concrete. The increased fluidity translates into higher slumping values, better adhesion of the concrete to reinforcing bars and better filling of void spaces in forms and the like. Products of this type are also sometimes referred to as "fluidifiers for fresh concrete" and "high range water reducers".
Water-soluble sulfonated melamine-formaldehyde resins have been found suitable for this purpose, e.g. as described in U.S. Pat. No. 4,272,430 to Pieh et. al. issued on Jun. 9, 1981. However, conventional methods of producing such resins have generally involved a condensation step carried out at a low (highly acidic) pH value in order to produce a high molecular weight product. Unfortunately, as a result of this, the final resin product contains a large percentage, e.g. 10% or more, of inorganic salt (usually sodium sulphate) arising from the required neutralization of the acidic reaction mixture after the condensation step has taken place. The presence of such salt potentially can adversely affect the physical properties of the concrete when the resin is used as a superplasticizer and may reduce the desired fluidizing properties of the additive, but is difficult and expensive to remove the salt from the liquid resin product by methods such as precipitation filtration or dialysis.
This problem has been addressed in U.S. Pat. No. 4,444,945 to Sheldrick issued on Apr. 24, 1984 and a process is disclosed that avoids a final condensation step at low pH. According to the patent disclosure, melamine, formaldehyde and bisulfite are condensed in a first stage in an aqueous solution at atmospheric reflux and pH 8-10 for 1 to 5 hours. In a second stage, condensation is continued at pH 6-7 and atmospheric reflux at pH 6-7 for 2-6 hours. It is stated that this produces low-salt containing aqueous solutions of melamine formaldehyde condensation products having anionic sulfo groups. We do not know whether this process is effective but it is anyway unsatisfactory because long reaction times are apparently required and the product may therefore be expensive. From the figures provided in the patent, it also seems that the dry resin products still contain about 5% by weight of inorganic salt, and it would be advantageous to be able to reduce the amount even further.